With increasing automatic control in various technical fields, optical scanners have found a plurality of different applications. Optical scanners are being used in areas such as optical storage, optical data communication, optical measuring tools, and laser printing.
As of today commonly used are bulky mechanical scanning systems employing a single laser or a laser array moved by an electro-magnetic mechanism.
Efforts aiming at improved performance requiring optical scanning systems without mechanical elements and the great demand for further miniaturization, have led to the design of scanning devices using acousto-optic interaction. In these scanning devices a surface acoustic wave (SAW) interacts with an incident light beam and deflects it out of its propagation direction. Two different types of acousto-optic interaction can be employed to deflect a light beam out of its original direction.
Most of the devices known in the art employ scanning mechanisms based on the acousto-optic Bragg interaction, schematically shown in FIG. 1. In these systems an incident light wave 10 (wavevector k.sub.opt), guided in an optical waveguide layer 11, interacts with an acoustic surface wave 12 (wavevector k.sub.a), as illustrated in FIG. 1. The surface acoustic wave generates periodic ondolations 13 at the waveguide surface and induces a periodic variation in the index of refraction of the surface and the area below by photo-elastic effects. Incident guided optical waves 10, as they interact with the region of induced fluctuation in the index of refraction, will be diffracted provided that proper phase matching conditions exist. The deflected light wave 14 (wavevector k.sub.s), propagates in the plane unfolded by the vectors k.sub.opt and k.sub.a. The angle alp.sub.B of deflection within the x-z plane is very small and the resolution is low. A portion 15 of the incident light wave 10 proceeds straight through the traveling grating reducing the intensity of deflected wave 14.
A detailed description of a deflector using acousto-optic Bragg interaction is given in "Thin Film Acoustooptic Devices", Proc. IEEE, Vol 64, pp. 779, 1976, published by G. Lean et. al.
Different patents, two of them are listed below, relate to optical pickup heads, using acousto-optic Bragg interaction for the lateral correction of tracking-errors:
U.S. Pat. No. 4,797,867, "Pick-up Head For Optical Information Storage Disk", and PA1 U.S. Pat. No. 4,425,023, "Beam Spot Scanning Device".
The fact that the Bragg diffraction condition has to be satisfied limits the angle of deflection. In addition it is not possible to reduce the 0-th order diffracted light portion 15, which goes straight on, as shown in FIG. 1.
Another kind of acousto-optic interaction is the colinear interaction. As shown in FIG. 2 and FIG. 3, an optical light wave 20 (wavevector k.sub.opt), propagating in a waveguide layer 21, interacts with a colinear surface acoustic wave 22 (wavevector k.sub.a) deflecting light wave 20 out of the x-z plane into a substrate 23. The deflection angle &alp..sub.c, between light wave 20 (k.sub.opt) and deflected light wave 24 (k.sub.s), of the colinear interaction system is bigger than alp.sub.B of a Bragg interaction system, due to the increased bandwidth of the acoustic wavelength .lambda. in contrast to the bandwidth of Bragg interaction systems.
Experimental demonstrations of the colinear acousto-optic interaction were first reported by Kuhn et. al., "Optical guided wave mode conversion by an acoustic surface wave", Appl. Phys. Lett., Vol. 19, pp. 428-430, 1971.
Additional experimental results, published by F. Gfeller, "A colinear thin-film acousto-optic scanner", J. Phys. D: Appl. Phys., Vol. 10, pp. 1833-1845, 1977, relate to systems schematically shown in FIG. 4 of the present application, involving a glass film 31 and a quartz substrate 35. This article, and particulary FIG. 8 thereof, is the nearest prior art known to the applicant and describes devices operating in the continuous wave-mode (CW-mode) such that the whole interaction path of length L is filled with an acoustic wave 32 (wavevector k.sub.a) of constant frequency as shown in FIG. 4. Light wave 30 (wavevector k.sub.opt) interacts with the SAW 33 over length L which is generated from a transducer 36. The angle alp.sub.cw depends on the wavelength .lambda. of the SAW 33. Controlling the shape of the deflected wave, represented by the parallel outer rays 34.1 and 34.2 in FIG. 4, is not possible because the device is operated in CW-mode. The optical resolution of devices operating in the CW-mode is limited since the light wave interacts with a "grating" having the length of the whole interaction path. The deflected beams are not forming single circular spots but smeared lines.
Hithereto it has not been shown how the shape of a deflected light wave can be controlled without expensive external lenses and optics, and how to reduce stray beams if a light wave from outside of the device is coupled into the device.